1. Field of the Invention
This invention relates generally to discharge machining equipment of machining a workpiece by electric discharge into desired dimensions and surface roughness, and more particularly to a machining control device for discharge machining equipment adapted so that machining can be effected in a short machining time with reproducible machining accuracy and surface roughness.
2. Description of the Prior Art
As machining speed is increased in discharge machining, machining accuracy and surface roughness tend to be deteriorated, and conversely as machining accuracy and surface roughness are improved, machining speed tends to be lowered. In order to realize machining in the shortest possible time while maintaining the dimensions and surface roughness of the workpiece, therefore, it has heretofore been practiced that machining conditions (the magnitude and duration of discharge current, etc.) are determined in accordance with the progress of machining, and appropriate machining conditions are selected stepwise for each machining stage from the start of machining to the end of machining.
Now, the prior art will be described in more detail, referring to FIGS. 3 and 4.
FIG. 3 is a diagram of assistance in explaining the progress of discharge machining. The machining state 1 shown on the right side of the figure illustrates the state where as machining proceeds under the machining conditions 1 in which rough machining is carried out at high speed, the electrode has been fed from the reference position to the position of an electrode feed Z.sub.1.
In the prior art, when machining proceeds under a machining condition 1 to the machining state 1 shown in the figure, machining is interrupted temporarily, and machining conditions are changed to a machining condition 2 involving a slightly lower machining speed and slightly improved surface roughness. Under the machining condition 2, machining is resumed until the electrode is fed to a predetermined feed Z.sub.2. Then, machining is temporarily interrupted, and machining conditions are changed to a machining condition 3. In this way, machining conditions are sequentially switched over as machining proceeds. A machining state m shown in the figure illustrates a machining state in the course of the machining sequence. As shown in a machining state n in the figure, the electrode is fed until a predetermined feed Z.sub.n is reached under the machining condition n as the final machining stage where satisfactory surface roughness can be achieved though machining speed is extremely low. In the machining state n, the discharge gap is .delta..sub.n, and the roughness of the workpiece surface is K.times.R.sub.n, and the workpiece is machined up to a predetermined position into a predetermined surface roughness. In this way, the prior art is such that machining is temporarily interrupted to sequentially switch over machining conditions so as to realize a short machining time and machining the workpiece up to a predetermined depth into a predetermined surface roughness.
As shown in FIG. 3, the surface of the workpiece during discharge machining has a topmost roughened layer having irregularities due to discharge energy, and a heat affected zone beneath the topmost roughened layer. In the gap between the electrode and the workpiece surface, suspended are chips generated by discharge. The particle size, density, etc. of chip may affect discharge machining.
FIG. 4A illustrates the state of power turning on and off across the electrode and the workpiece. S.sub.ON indicates the ON duration, and S.sub.OFF the OFF duration. FIG. 4B shows changes in discharge voltage pulses applied across the electrode and the workpiece. Since discharge does not start immediately after power is turned on (S.sub.ON), impedance between the electrode and the workpiece is high, and a high voltage V.sub.1 is produced across both. As discharge starts, the voltage across both drops to a lower discharge voltage V.sub.2. Consequently, the start of discharge current flow can be known by setting a voltage V.sub.d between the voltages V.sub.1 and V.sub.2 as the threshold value, and detecting discharge voltage falling below the threshold value. T.sub.ON is the duration when discharge current keeps flowing, while T.sub.OFF is the duration from the time power is turned off, to interrupt discharge current to the time power is turned on again.
The prior art described above has the following problems.
First, discharge machining having satisfactory reproducibility cannot be expected.
Although there are some formula to calculate the electrode feed Z.sub.n in accordance with the progress of discharge machining, the electrode feed Z.sub.n calculated from the formula does not warrant satisfactory reproducibility for discharge machining. The electrode feed Z.sub.n has heretofore been calculated by substituting in the following equation the data obtained by measuring in advance the machining gap value and the machined surface roughness value by changing any machining conditions among predetermined machining power conditions (discharge current, the ON duration S.sub.ON, and OFF duration S.sub.OFF of power). EQU Z.sub.n =Z-K.sub.t .times.R.sub.n -K.sub.f .times..delta..sub.n -.gamma..sub.n .times.(Z.sub.n -Z.sub.n-1)
where K.sub.t and K.sub.f are prescribed coefficients; Z the distance between the reference point and the workpiece as a target; R.sub.n a value indicating surface roughness; .delta..sub.n a gap; and .gamma..sub.n electrode consumption ratio.
In actual discharge machining with the prior art, however, there is a difference, caused by the presence of discharge chips, between the size of the machining gap when machining conditions are sequentially changed and the size of the machining gap in the machining performed to obtain the afore-mentioned data. For this reason, discharge machining with satisfactory reproducibility cannot be expected from the above equation.
In the prior art, moreover, discharge current is temporarily turned off when changing machining conditions (particularly, discharge current). This causes discharge chips suspended in the machining gap to be scattered, making the state of machining gap undesirable for discharge machining under the next machining conditions. In other words, the absence of chips in the machining gap makes if difficult to resume discharge in the same machining gap. To resume discharge, therefore, the discharge gap must be reduced prior to the changeover of machining conditions. This, in turn, could lead to machining to an unexpectedly large depth as machining proceeds with the reduced discharge gap. This unwanted condition may happen randomly with different workpieces which are to be subjected to the same machining, resulting in deteriorated machining accuracy and workmanship, or discharge machining with poor reproducibility.
Second, it takes much time to position the electrode to ensure a proper gap with the prior art. This leads to extended machining time, the failure of obtaining desired surface roughness, and the failure of metal removal.
The original purpose of sequentially changing machining conditions is to reduce machining time. The prior art, however, can reduce machining time to some extent, but can still make considerable time, as will be described later. This is because the prior art, which relies on a limited number of changeover steps, tends to involve large differences in machining conditions among changeover steps. (For example, when discharge current is changed from 10A to 8A in a low consumption region, the energy is almost halved, and the size of discharge chips greatly varies.) For this reason, when changing machining conditions to the next step, it takes some time until discharge machining reaches a level expected under that machining condition step.